Method for increasing the milk let down in mammals

ABSTRACT

The use of long-acting oxytocin analogs, characterized chemically by replacement of the disulfide bridge by a thioether bridge, in safely increasing the milk products in mammals is disclosed. Of particular importance in the use of these compounds is the safety factor, that is, the absence of dangerous side effects at dosage levels required for increasing lactation. An increase in milk production can be accomplished with one or, at most, two single injections.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of co-pending application Ser. No. 31,062 filed Apr. 18, 1979, now U.S. Pat. No. 4,237,119 which is a continuation-in-part of Ser. No. 931,134, filed Aug. 4, 1978, which, in turn, is a continuation of application Ser. No. 813,665, filed July 7, 1977, both of which are now abandoned. The contents of these prior applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to a pharmacological means of increasing milk production and in particular treating agalactia post partum and lactation failure in mammals, particularly farm animals.

It is known that the natural hormone oxytocin can produce contractions in the collecting ducts of the mammary glands with the resulting ejection of milk.

Agalactia post partum is characterized by partial or complete lactation failure one to three days after parturition. Other symptoms such as elevated rectal temperature, depression, reduced appetite and mastitis are often observed.

Oxytocin is widely used as part of the therapy of the affected mammals in effecting milk production. The biochemical half life of injected oxytocin is very short. Treatment with oxytocin has only a temporary effect. Treatment therefore requires several injections which can produce undesirable side effects in the animals. A better treatment would be the use of a long action composition which provided a similar response over an extended time period.

Controlled parturition in cattle has been a subject of veterinary investigation for over a dozen years. Aside from possible clinical reasons for inducing labor before its spontaneous onset, in large dairy herds, such a regimen could have economic significance if births could be planned to occur at manageable rates for the available personnel during their normal working hours, So far, three categories of substance have been tested for this purpose: steroids such as dexamethasone or flumethasone, prostaglandins and oxytocin.

Oxytocin (OT) in moderate dosage produces regular contractions of the pregnant uterus, but for very short periods of time after single doses and in addition vasoconstricts the blood supply to the uterus. Attempts to prolong the effect by giving excessive doses can result in uterine tetany or tachyphylaxis, both of which, as with prostaglandins, endanger the liver of mother and fetus. In human use, OT is given in continuous infusion at 1 ml/min of a solution of 20 IU/l.--which represents a rate of about 40 ng or pmol/min--in 5% glucose, but constant monitoring of uterine contractions and fetal heartbeat are required to prevent tetany and fetal damage. Reports of use in bovine obstetrics vary from a failure to induce labor with i.v. or i.m. injection of 100 IU (=about 0.2 mg.) to successful induction with i.v. infusion of only 4-5 IU over 1 hour (but in only 3 animals). A long-acting analog of OT could, thus, obviate such difficulties with tetany and tachyphylaxis, and in particular the practical impossibility of routine setting up of long term infusions in large animals, by accomplishing the desired effect with one or, at most, two, separate i.v. injections.

It would be of advantage to have an oxytocic drug which would, after one or, at most, two single injections, result in an oxytocic response starting within minutes of administration and lasting for several hours--of sufficient duration to cover the course of accelerated labor. Analogs of oxytocin have been described with a prolonged oxytocic activity such as 1-deamino-oxytocin, 1-deamino-1,6-dicarba-oxytocin and derivatives thereof (Kobayashi A., Hase S., Kiyoi R., Sakakibara S.: Bull.Chem.Soc. Japan 42:3491 (1969), Walter R. Yamanaka Y., Sakakibara S., Proc.Nat.Acad.Sci. U.S. 71:1901 (1974). Di-carba oxytocin analogs, however, in which both sulfur atoms have been replaced by methylene groups, although they have prolonged activity, show such low potencies that very large doses would be required (Barth T., Krejci I., Kupkova B., Jost K., Europ.J.Pharmacol 24:183 (1973); Barth T., Krejci I., Vaneckova J., Jost K: Europ.J.Pharmacol. 25:67 (1974)).

There is also a relatively short-acting oxytocin analog[ 2-Tyr(OMe)]-oxytocin, which, in comparison with the natural hormone, has one tenth the contractile activity on the myometrium, but its vasoconstrictor activity in the same organ is decreased to one hundredth (Hodr J., Stembera Z. K., Brotanek V., Rudinger J., Vondracek J.: The influence of methyl-oxytocin on glycide metabolism of mother and foetus, In "Intra-uterine Dangers to the Foetus," Excerpta Medica, Amsterdam, 1966, p. 445) so that if 10 times as much [2-Tyr(OMe)]-oxytocin as oxytocin itself (w/w) is administered, the same expensive force from the uterus can be attained without a toxic, hypoxic effect on the fetus or induction of metabolic acidosis in the mother during labor. This short acting analog is commercially produced and in clinical use (Methyloxytocin SPOFA) and its effectiveness has been demonstrated (Bartschi R., Huter J., Romer V. W., Geburtshilfe u. Frauenheilkunde 32:826 (1972)). Just as with oxytocin, induction of labor with this analog requires a continuous infusion and careful following of uterine contractions and cardiac action of the fetus in order to prevent damage to the latter.

In the sow, as in all other mammals, the biochemical half-life of injected oligopeptide hormones such as oxytocin is only a few minutes and the duration of the myometrial response to single injections is only slightly longer. As in other mammals, the sensitivity of the myometrium in the sow is increased under the influence of estrogen both in vitro and in vivo. While the duration of uterine response can be prolonged by giving large doses of oxytocin, one thereby runs the risk of uterine tetany (particularly during labor) and tachyphylaxis. Ideally the most physiological approach to prolonging oxytocin action until the required clinical task is accomplished would be to administer it by continuous i.v. infusion as in human obstetrics. This is hardly possible under ordinary conditions of veterinary medical practice with large animals. One alternative solution would be a very long-acting analog of oxytocin so that single, or at most two separate, injections could cover several hours of pharmacological activity.

SUMMARY OF THE INVENTION

It is the primary object of the present invention to provide a drug which, after application, maintains its activity for a long period.

It is another object of the present invention to provide a method of increasing milk production in mammals by administering the drug described in the foregoing object.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Consistent with the foregoing objects, this invention is based on a long-acting structural analog of 1-deamino-oxytocin in which the disulfide bridge between residues 1 and 6 is replaced by a thioether one, as shown in the general formula I: ##STR1## where X=H or CH₃ and Y=CH₂ S or SCH₂. The dosage varies from 1 ng to 10 mcg/kg body wt. It has now been determined that the above substances can be used for:

(1) induction and completion of labor in women or in pregnant animals before the natural onset of labor, with a closed cervix, (2) acceleration of labor in women and animals after a physiological onset, but with slow progress,

(3) accomplishment of (1) and (2) above without signs of uterine tetany or fetal anoxia, i.e., with an improved safety factor for both fetus and mother, and

(4) facilitating milk production in dairy animals.

These analogs of 1-deamino-oxytocin were prepared by classical synthetic methods in solution (Jost. K., Collect. Czech.Chem.Commun. 36, 218 (1971); Jost K., Sorm F., ibid. 36:234 (1971); Rudinger J., Jost K., Czechoslovak patent 123272 (PV 2429-61); Fric I., Kodicek M., Prochazka Z., Jost K., Blaha K., Collect. Czech.Chem.Commun. 39:1290 (1974); Jost K., Barth T., Krejci I., Sorm F., Czechoslovak patent 149028 (PV 1122-71)) and were available for experiments as freeze-dried powders. Chemical, physicochemical and pharmacological properties of some examples are listed in Table 1. All of these substances are easily soluble in water and solutions for injection were prepared containing 0.1 mg/ml normal saline, under sterile conditions, at pH about 4. Such solutions were kept at 5° C. until use.

                                      TABLE 1                                      __________________________________________________________________________                      Analysis              R.sub.f                                           Formula                                                                               % C                                                                               % H                                                                               % N Optical rotation                                                                           TLC   Biological activities                                                          IU/mg)                            Substance (m.w.) (calculated/found)                                                                       (solvent)                                                                               K  SBN/SBA                                                                              A  B  C  D  E  F                  __________________________________________________________________________     Ia        C.sub.44 H.sub.67 N.sub.11 O.sub.12                                                   50.52                                                                             7.23                                                                              14.73                                                                              -71.7°                                                                           2.15                                                                              0.20  1899                                                                              1206                                                                              1127                                                                              581                                                                               604                                                                                24                X=H, Y=CH.sub.2 S                                                                        S.4H.sub.2 O                                                                          50.42                                                                             6.62                                                                              14.61                                                                              (water)     0.35                                              (1046)                                                               Ib        C.sub.44 H.sub.67 N.sub.11 O.sub.12                                                   53.26                                                                             7.01                                                                              15.53                                                                              -84.5°                                                                           1.95                                                                              0.20   922                                                                              2792                                                                               571                                                                              755                                                                               456                                                                               118                X=H, Y=SCH.sub.2                                                                         S.H.sub.2 O                                                                           53.13                                                                             7.09                                                                              14.90                                                                              (1M CH.sub.3 COOH)                                                                         0.35                                              (992)                                                                Ic        C.sub.45 H.sub.69 N.sub.11 O.sub.12                                                   52.78                                                                             7.18                                                                              15.05                                                                              -69.0°                                                                           3.15                                                                              0.35   10                                                                                40                                                                               --  67                                                                                35                                                                               0.9                X=CH.sub.3, Y=CH.sub.2 S                                                                 S.2H.sub.2 O                                                                          52.29                                                                             6.90                                                                              14.88                                                                              (1M CH.sub.3 COOH)                                                                         0.36                                              (1024)                                                               __________________________________________________________________________      K = partition coefficient between 2butanol and 0.05% acetic                    SBN = 2butanol, 25% ammonia, water (85:7.7:7.5);                               SBA = 2butanol, 90% formic acid, water (75:13.5:11.5).                         A = isolated rat uterus,                                                       B = in situ rat uterus,                                                        C = vasodepression in the cock,                                                D = isolated rat mammary gland,                                                E = lactational activity in vivo                                               F = antidiuretic in the rat.                                                   TLC = thinlayer chromatography.                                          

Use of these substances has a number of advantages. Both for the health of the newborns and the cost in materials and time in care, it is advantageous to have a long-acting oxytocic effect which can rapidly initiate the entire process of labor and birth with an accelerated, physiological course with single injections of 5 mcg/kg body wt., without the danger of inducing uterine tetany and fetal anoxia which can complicate the use of other oxytocics for this purpose. In the case of calves it is important for their health that they get colostrum as a source of maternal immonoglobulins as soon as possible after birth. This requires personnel and is simpler to carry out if the calves are born in the daylight hours of a normal working day, which the substances referred to in this invention enable. The chemical basis of the claimed activities are based on (a) the absence of an N^(alpha) -amino group of 1-cysteine which prevents aminopeptidase cleavage of the molecule into inactivity, (b) replacement of the disulfide bridge by a thioether one, which eliminates disulfidase cleavage of the molecule into inactivity and (c) if desired, replacement of the p--OH group of 2-tyrosine by a methoxy group. With the prolonged activity thus produced, labor can be induced early in the morning on a working day and birth can take place in the afternoon of the same day when combined with dexamethasone or prostaglandin pre-treatment, or with injection the evening before and birth the next working day if used without any other pre-treatment. This can be used several days to one week before the expected onset of spontaneous labor. In swine, labor is prolonged with large litters and frequently the last piglets to be born are anoxic and either die or the mother kills them. Compounds used in the present invention, in the same dosage also cause prolonged, rhythmic contractions of the sow's uterus with no signs of tetany, and so can be used to accelerate labor and increase the percentage of viable, healthy young. The preferred range of dosage is from about 10 ng to about 100 μg. per Kg. of body weight. A more preferred range is from about 1 μg to about 10 μg per Kg. The substance is administered in solution in any carriers such as saline or water.

It is known that low dosages of compounds of the formula I have prolonged contractile effect on the mammary glad of the rat (Barth, T., Jost, K., Rychalik, I.: Endocrinology exp. 9,35 (1975); Barth, T., Flegel, M., Jost, K.: Endocrinology exp. 10,65 (1976) but not until the present invention was it known that one injection can produce milk ejection is unanesthetized animals especially cows.

Dosage of compounds of formula I suitable to increase milk production vary from about 10 ng to about 100 μg per Kg of body weight and preferably from about 1 μg to 10 μg per Kg of body weight: In low dosages of about 1 μg/Kg given as a single injection there is prolonged evacuation of the udder and increased milk delivery.

EXAMPLE 1

The milk-let-down effect of 1-desamino-1-monocarba-(2-Tyr(OMe))-oxytocin (hereinafter noted as dCOMOT) was tested in healthy lactating sows. The milk-let-down effect of 0.6 mg of dCOMOT injected intravenously lasted for from 4 to 9 hours. Intramuscular injection gave approximately the same response.

Intraveneous injection of 5-10 I.U. of oxytocin to healthy lactating sows resulted in a rapid increase of the intramammary pressure which lasted for 5-10 minutes; 0.6 mg dCOMOT administered intraveneously gave the same initial response followed by a secondary oscillation in pressure which lasted for at least 4 hours.

EXAMPLE 2

A study was made on the effects of oxytocin and dCOMOT on 46 cows affected with hypogalactia or agalactia. The sows belonged to four different large herds. The feeding and management were conventional. All sows were examined on the first day the clinical symptoms were observed. After examination the sows were treated conventionally with antibiotics for 1-3 days. Twenty-five sows were given 0.6 mg dCOMOT i.v. or i.m. and 21 sows were given 30-50 I.U. oxytocin on the first day the clinical symptoms were observed.

Sows failing to nurse the piglets satisfactorily on the day after treatment were retreated with oxytocin. In addition to recording the clinical symptoms, including rectal temperature and mastitis, the following data were noted for each sow: parity, length of gestation, litter size and litter weight at examination/treatment, litter size and litter weight 7 days after 1st treatment. Clinical mastitis was defined as swelling and hardening of one or more udder glands.

In Table 2 the sows are listed according to the day of examination/treatment after farrowing. The clinical observations of the sows are listed in Table 3. Neither mean age nor gestation length differed between the two groups of treated sows. The rectal temperature was above 39.5° C. in approximately two-thirds of the sows. Incidence of mastitis was high among diseased sows.

                  TABLE 2                                                          ______________________________________                                         The Day of 1st Examination/Treatment                                                     Day of      1st day after                                                                             2nd day after                                 Treatment farrowing   farrowing  farrowing                                     ______________________________________                                         dCOMOT    8           13         4                                             oxytocin  6           12         3                                             ______________________________________                                    

                  TABLE 3                                                          ______________________________________                                         Clinical Observations of the Sows                                                              Treatment Group                                                Observations      dCOMOT    oxytocin                                           ______________________________________                                         Parity (litter number)                                                                           2.9       3.5                                                Gestation length (days)                                                                          115.8     115.9                                              Rectal Temperature (°C.)                                                                  39.61     39.56                                              Nos. of sows with rectal                                                                         14/24     14/21                                              temp. above 39.5                                                               Mastitis          19/25      8/20                                              ______________________________________                                    

Litter sizes are presented in Table 4. As is evident from this table no significant differences were recorded between the two treatment groups on the day of treatment or on the 7th day after treatment. Therefore the survival rate was the same within the treatment groups.

                  TABLE 4                                                          ______________________________________                                         Litter size (Mean ± s.d.) at day of treatment                               and at day seven after treatment.                                              Treatment  Day of Treatment                                                                            7 days after treatment                                 ______________________________________                                         dCOMOT     10.16 ± 1.65                                                                             8.92 ± 1.85                                         oxytocin   10.00 ± 1.95                                                                             9.10 ± 2.10                                         ______________________________________                                    

The mean piglet weight per litter within the two treatment groups on day of treatment and on day seven after treatment are presented in table 5. As is evident from this table the mean piglet weight was the same in the two treatment groups on the day of treatment. The piglets belonging to dCOMOT treatment group, however, gained more weight than the piglets belonging to oxytocin treatment group, during the observation period.

                  TABLE 5                                                          ______________________________________                                         Mean piglet weight per litter at day of                                        treatment and at 7 days after treatment                                                 Weight of piglets (kg)                                                Treatment  Day of treatment                                                                            7 days after treatment                                 ______________________________________                                         dCOMOT     1.44 ± 0.37                                                                              2.64 ± 0.62                                         oxytocin   1.40 ± 0.22                                                                              2.28 ± 0.44                                         ______________________________________                                    

None of the dCOMOT-treated sows failed to eject milk on the day after treatment. Four out of 17 oxytocin-treated sows did not eject milk and were retreated on the day after the 1st treatment. No side effects were observed after treatment in any sow independent of treatment.

EXAMPLE 3

250-300 g body wt female rats (n=5 for each substance) were pre-treated for 3 days with 50 mcg di-ethylstilbestrol and 25 mg progesterone injected once daily i.m. in order to fully "estrogenize" the animals and have them all in the same functional state of myometrial reactivity. They were then anesthetized with urethane and prepared for in situ measurements of uterine horn contractions using Grass force-displacement treansducers and polygraph recording. Table 2 shows that equipotent doses of 1-deamino-1-monocarba-[2-Tyr(OMe)]-oxytocin,1-desamino-oxytocin,[2-Tyr(OMe)]-oxytocin, and oxytocin itself, all in doses comparable to those used to induce labor in cows in Example 2 with the first of the three substances named above, gave the following results: (1) the longest duration of oxytocic action was associated with the monocarba analog, followed by the other substances in the order given above. (2) while neither of the two [2-Tyr(OMe)] substituted analogs showed any incidence of uterine tetany, long-acting 1-deamino-oxytocin and short-acting oxytocin both gave high incidences of uterine tetany in the equipotent doses used.

                  TABLE 6                                                          ______________________________________                                         Substance (dose)                                                                            A       B           C     D                                       ______________________________________                                         1-deam.monocarba-                                                                           65      (60 ± 5.3)                                                                              185   none                                    [2-Tyr(OMe)]--OT                                                                            ±6.3             ±18                                        (10 mcg/kg i.v.)                                                               (d COMOT)                                                                      1-deamino-OT 92      (65 ± 6.2)                                                                              46    60                                      (1 mcg/kg i.v.)                                                                             ±6.5             ±6.3                                       (d OT)                                                                         [2-Tyr(OMe)]-OT                                                                             68      (42 ± 5.4)                                                                              20.6  none                                    (10 mcg/kg i.v.)                                                                            ±7.4             ±2.3                                       (Me--OT)                                                                       oxytocin (OT)                                                                               90      (43 ± 3.2)                                                                              10.4  60                                      (1 mcg/kg i.v.)                                                                             ±4.2             ±2.6                                       ______________________________________                                          where A = amplitude of contractions as a % of maximal contraction induced      by a tetanic dose of 10 mcg/kg oxytocin at the end of the experiment in        each animal,                                                                   B = (numbers) = contraction frequency per 10 min. in %                         C = duration of the response in minutes                                        D = the presence of tetany in % of the animal group, as defined by a           single maximal contraction lasting more than 10 min.                           All numbers as means ± SEM, except for D.                             

EXAMPLE 4

These experiments were carried out on 13 pregnant `Czech red-spotted` cows ranging from para I to para V. Average pregnancy in this breed is 290 days in duration.

Nine of the animals were 278-290 days after insemination but actual active labor had not yet begun. These animals received no other medication (Table 7).

                                      TABLE 7                                      __________________________________________________________________________     Group I - effect of dCOMOT in non-pretreated cows.                             Age      dCOMOT dose                                                                              Duration of Labor                                                                        Day of pregnancy                                                                          Progeny           Placenta             No.                                                                               years                                                                             Para                                                                              1st  2nd  hrs.      (from insemination)                                                                       Sex  Body wt (kg)                                                                          Serum                                                                                deliver              __________________________________________________________________________     1  3  II 5    --   26        287        M    51     7.4   -                    2  4  II 5    --   34        287        F    42     7.4   -                    3  5  IV 5    --   22.5      290        M    51     7.4   +                    4  6  V  5    --   6         286        F    40     7.4   -                    5  5  IV 5    --   24.5      278        M    45     7.3   -                    6  3  II 5    --   10        281        F    43     7.4   +                    7  2  I  5    --   1.5       288        M    45     7.4   +                    8  2  I  5    --   7         290        F    39     7.3   -                    9  5  IV 3    2.5  15        287        F    45     7.4   -                    __________________________________________________________________________

                                      TABLE 4                                      __________________________________________________________________________     Group II - effect of dCOMOT in pretreated cows.                                Age      dCOMOT dose                                                                              Duration of Labor                                                                        Day of pregnancy                                                                          Progeny           Placenta             No.                                                                               years                                                                             Para                                                                              1st  2nd  hrs.      (from insemination)                                                                       Sex  Body wt (kg)                                                                          Serum                                                                                delivery             __________________________________________________________________________     1  4  II 5    --   0.7       280        M    44     7.4   -                    2  3  I  5    --   8.2       273        F    40     7.4   -                    3  4  II 5    2.5(7 h)                                                                            8.8       280        F    48     7.4   -                    4  3  I  5    --   8.6       275        F    35     7.4   -                    __________________________________________________________________________

Four of the animals were estimated to be near to term (cf. Table 4). 48 hours before the actual experiment they received 20 mg dexamethasone i.m. in an attempt to prepare the uterine cervix for active labor. In this group, in addition to close clinical follow-up, measurements were also made of: body temp., hemoglobin (Hb), glucose, total protein, inorganic P, Ca and Mg concentrations in the blood, both in the cow during the experiment and in the calf after birth, all by standard methods.

The standard i.v. (jugular) injection dosage of dCOMOT was 5 mg. In those cases in which labor lasted longer than 6 hours (cf. Results) in second injection was given.

The first group of 9 cows was managed under field conditions--in the barns of local farms. The four animals under more intensive study were kept in stalls set aside in a research building for one week prior to the onset of the experiment and for 3 weeks following delivery for further postnatal observations.

GROUP I--9 NON-PRETREATED COWS

All the animals were in the range of term pregnancy--278-290 days from insemination (mean 286±2.0 SEM) (Table 2)--and clinically their status corresponded well: loosening of pelvic joints and connective tissue, edematous and dilated state of the vagina and vulva and filled state of the udders. Vaginal and rectal examination showed, however, that in no case were there regular uterine contractions related to the onset of labor and the fetal membranes were intact. The mean width of the external os at the start of the experiment was 5±1.4 cm, range 2-10 cm).

In all cases, injection of dCOMOT was followed by the onset of regular uterinne contractions and increased tension in the udder (with milk ejection either at rest or during Valsalva "bearing down"). Of great interest was the fact that the course of the induced labor was highly physiological--the rate of contractions started at about 1/10-15 min and slowly accelerated, while the intensity of the contractions apparently also increased with time. There was no evidence of uterine tetany whatsoever. In the most dilated cow (10 cm external os at the time of dCOMOT injection), delivery was complete 1.5 hours after injection. All the calves were healthy and continued to develop normally in the post-natal period. Mean duration of labor was 14.25±5.1 (SEM) hours from the dCOMOT injection.

GROUP II--4 DEXAMETHASONE PRETREATED COWS

This group ranged in duration of pregnancy from 273 to 280 days (mean 277±1.8)--somewhat further from term than Group I. In all cases the external os was closed when, 48 hours before dCOMOT injection, they received 20 mg dexamethasone i.m. At the time of dCOMOT injection, the external os ranged from 4 to 10 cm, mean 6±1.5. In all cases, as above, active labor started within minutes of dCOMOT injection, not before. The mean duration of labor was only 4.3±2.1 hours, range 1.52 (external os 10 cm at injection) to 8-9 hours.

As in Group I, here as well, all calves were healthy and remained so postnatally, showing no signs of hypoxia at birth. In no cases was uterine tetany observed during the induced labor.

The sensitivity of the myometrium to OT is fairly low in the non-pregnant uterus (in comparison with vasopressin, for example and for the first third of pregnancy. Sensitivity increases rapidly as term approaches, and so is clearly a function of myometrial changes induced, among other things, by increasing levels of secreted steroids. As pointed out earlier, attempts at prolonging OT action by increasing dosage carry with them the danger of producing either uterine tetany or tachyphylaxis or both sequentially. Such a result is extremely dangerous for both mother and fetus. Of great practical and perhaps theorectical interest in the present work and other related observations is that a long-acting OT such as dCOMOT has not yet shown any signs of producing tetany or tachyphylaxis despite large doses.

Combination of dCOMOT with dexamethasone clearly gave the best results since the shortest times for all stages of labor were recorded in Group II. It would appear that using such a combination, injection of dCOMOT at, say, 5:00 AM would result in the birth of a healthy calf within the same 8-hour work shift on the same day. Were dCOMOT to be used alone so as to avoid steroids, the peptide would have to be given the afternoon before the day of delivery.

EXAMPLE 5 Experiment I--in vitro in sows

The stage of the estrus cycle was determined at the slaughter house by the appearance of the ovaries--at estrus the latter contain large follicles (8-10 mm) combined with small corpora lutea (3-4 mm). The reproductive organs were removed from four sexually mature females in heat within a few minutes of sacrificing and were rapidly transported to nearby laboratories. A number of strips 20×4 mm were cut longitudinally from the uterine horns suspended in Tyrode solution at 5° C. The strips were then mounted in 40 ml cuvettes in isolated organ baths. The bath fluid (Tyrode solution bubbled through with 5% CO₂ in O₂) circulated continuously from below at a constant rate of 1 ml/min and was maintained at a temp of 37° C. by a thermostat. Contractions were recorded by a force-displacement transducer (Grass FTOC3) connected to a Polygraph (Model 79C, Grass Instrument Co., Quincy, Mass.). It took about 2 hours as a rule for the strips to reach a balanced baseline state of activity. Only after the latter had been attained were the following substances added to the medium alternately, in increasing dosage: synthetic oxytocin (OT) (Partocon®-Ferring, Malmo, 10 IU/ml) and 1-desamino-1-monocarba-[2-Tyr(OMe)]-OT (dCOMOT) (substance supplied by Drs. K. Jost and J. H. Cort, ampuled by Ferring at 0.2 mg/ml physiological saline, pH about 4).

Spontaneous activity and responses to OT and dCOMOT were determined. The baseline contraction rate was 6-8/10 min. All strips responded to OT, initially with an increase in baseline tone and thereafter an increase in contraction rate. Duration and intensity of response varied between strips independently of dosage (2.5-25 mIU/ml medium) (the equivalent of 5-50 ng OT/ml) with duration varying from 15 to 40 min.

dCOMOT medium concentrations were varied from 500 to 5000 ng/ml and the pronounced responses ranged from 30 to 120 min without relation to the above concentration range. The response was more related to an increase in contraction rate than to an increase in baseline tone. It was noted that there was a tendency to waves of response during the prolonged effects of dCOMOT. Despite the large doses used, the strips did not go into tetany.

EXPERIMENT II--IN VIVO IN SOWS

Four gilts were used. The experiments stated about 24 hours after onset of their second or third estrus. Their cycle duration was in the range 19-22 days. They were anesthetized by i.v. administration of about 15 mg/kg Na thionembutal (in 5% (w/v) solution)--additional doses were required during the experiment.

The uterus was exposed through a midline abdominal incision. A recording balloon-tipped catheter was inserted into each uterine horn through a small incision about 10 cm from the uterine body (Clay-Adams catheters, PE 90-0.86 mm i.d.×1.27 mm o.d., with a micro-balloon sealed over the tip with a double tie of 0000 silk suture). The recording system was filled with saline and each catheter was affixed to the uterine wall with 000 silk suture to prevent motion of the balloon in the lumen. The catheters exited through the mid-abdominal incision and were connected to P-23AC pressure transducers, thence to 7PL DC preamplifiers. When we were satisfied that the recording system was functioning the abdominal incision was closed about the catheters.

Spontaneous uterine activity was recorded for at least one hour, usually two hours, before test substances were administered (OT and dCOMOT as above) i.v.

In both experiments, since orientation results only were sought and limited amounts of dCOMOT were available, a dCOMOT:OT ratio of 100 was used as a more judicious investment of drug in a large animal should the lower ratio prove ineffective. Unfortunately there was not enough material for a full scale dose-response investigation.

The baseline contraction rate was 6/10 min. OT was given i.v. in doses of 1 to 5 IU (equivalent to 2-10 ug). The response to OT involved initially a small increase in tone followed by an increase in contraction rate from 6/10 to 8/10 min. In one case this was also accompanied by an increase in contraction amplitude. Over the dosage range given, response duration was approximately 10 min. dCOMOT was injected in the dose range 0.2 to 1.0 mg. The response ranged from 60 to 120 min in duration between gilts. However, there was no significant dose response in the dose range used. Initially, a small 2 min. increase in tone was demonstrated. Thereafter the most pronounced aspect of the response was the increased contraction rate (from 6/10 to 8-10/10 min). Again there were waves of reaction seen after dCOMOT injection, but no tetany despite the large dose. 

What is claimed is:
 1. A method for increasing the milk let down in mammals which comprises administering an effective amount of a compound of the formula ##STR2## where X is H or CH₃ and Y is CH₂ S or SCH₂.
 2. A method according to claim 1, wherein said effective amount varies between about 10 ng and about 10 μg/kg of the body weight of the mammal.
 3. A method according to claim 2, wherein said effective amount is from about 1 μg to about 10 μg per Kg.
 4. A method according to claim 1, comprising administering said compound by injection as a soslution in a pharmaceutically acceptable carrier. 